LAKE CHELAN
of a river may be a few feet only or a few scores of feet; that of
a glacier may be thousands of feet. It is this greater size, vol
ume, and weight which makes glacial ice behave like water. In
such large masses ice is plastic, accommodating itself to inequal
ities of its bed, flowing with some freedom, spreading out and
contracting, much as water does.
A word of caution must here be interpolated. The channel
of a river, in which its water flows, must not be confused with
its valley, which it drains. The above comparison refers to the
channel of a river, not to its valley.
Glaciers in mountain regions commonly head in amphithea
ters or cirques-basins lying directly at the heads of canons,
under the shadow of the summit cliffs. An amphitheater is
surrounded on three sides by vertical walls or steep slopes, down
which the ice and snow slide in avalanches, accumulating in
the bottom. The effect is precisely like that of a waterfall.
The falling snow and ice dig a hollow or depression at the foot
of the steep descent just as water does. Such amphitheaters
are found at the heads of all glacial gorges in high mountains,
and today are found to contain small alpine lakes in place of
the ice which once occupied them. From its head in the amphi
theater the glacier moves down the gorge, scouring and cutting
the bottom and sides as it travels. The ends of the mountain
spurs are planed off instead of being trimmed to sharp, angular
points, as is done by streams in gorges cut by them. If the
bottom of the canon be uneven, if it contain abrupt elevations
and depressions, the glacier flows over them as water would
flow over similar obstacles in its channel, gradually cutting
them away. Where the descent becomes abruptly steeper the
ice, in bending to follow the surface, is commonly cracked, form
ing a network of crevasses, making travel over its surface very
difficult and dangerous.
Where the main glacier is joined by a branch, the bed of the
branch is commonly found to be at a higher level than the bed
of the main glacier, because being larger and heavier the main
glacier has greater cutting power; indeed, in many cases the
beds of small branches are hundreds, or even thousands, of feet
higher than that of the main glacier to which they are tribu
tary. The parallelism between the glacier and the river in their
channels is further illustrated by this fact. The surface of the
ice in the main glacier and in the branch must have been at the
same level, although the bottoms, as stated above, differ greatly
419